1. Field
The present invention relates generally to communications, and more specifically, to a system for selecting optimal transmission formats either for a single user or for simultaneous transmissions to multiple users.
2. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. As used herein, the term xe2x80x9ccellularxe2x80x9d system encompasses both cellular and personal communications services (PCS) frequencies. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project xe2x80x9c3GPPxe2x80x9d, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
The telecommunication standards cited above are examples of only some of the various communications systems that can be implemented. But problems exist amidst them all. Namely, multiple users must share limited system resources. In accordance with the actual system implementation, resources such as frequency bandwidth, time, transmission power, or spreading code assignments are typically shared by multiple users within the system. When allocating these system resources, the issues of fairness and efficiency must be considered by the service provider. In a FDMA system, the system bandwidth is divided into many frequency channels and each frequency channel is allocated to a user. In a TDMA system, the system bandwidth is divided into many time slots and each time slot is allocated to a user. In a CDMA system, the system bandwidth is simultaneously shared among all users by using spreading codes, wherein each user is assigned a spreading code.
In systems that can transmit data traffic in packetized formats, such as TDMA and CDMA systems, efficient scheduling of multiple users is a key aspect of system performance. In a typical TDMA system, only one user can be scheduled in a slot. A slot is a unit of time that carries a predetermined number of bits. The size of the slot can vary according to system design constraints. The scheduling of data for transmission in slots is typically based on whether data is designated for a user and whether the quality of the channel is within acceptable parameters. However, there are several reasons why this scheduling method is inadequate for optimized system performance.
One inefficiency problem with this scheduling method occurs whenever the amount of data designated for a user is smaller then the data transport capability of the system. If the quality of the channel is very high, then a large amount of data bits can potentially be transported in the assigned time duration. However, if the actual data that is to be transported is less than the potential data capacity, then the xe2x80x9cfat pipexe2x80x9d is inefficient in terms of system throughput. It should be noted that system throughput is determined by the rate at which the original information bits is actually received, which differs from the rate of the transmitted bits within the slotted channel. Information bits are encoded, interleaved and modulated before transmission, so that the number of transmission bits that actually travels over the channel varies greatly from the original number of information bits.
Another inefficiency problem arises from quantization loss. In order to simplify the implementation of the communication system and to reduce the signaling overhead, various parameters are quantized with a limited number of quantization levels. For example, the number of payload bits that are transmitted in a packet, the modulation format and the frame duration are parameters that are typically rounded to allowed quantization levels. Due to the quantization nature of transmit formats, there is almost always a gap between the number of information bits that are actually transmitted in a slot and the number of bits that the system can support if there were no quantization. For example, if a system has data rates of 9.6 kbps and 192, kbps, then the system can transmit at only one of these two rates. Suppose a channel to a user can support 15 kbps. However, in order to ensure success, the system will assign a transmission rate of 9.6 kbps due to the quantization of data transmission rates. Hence, there is a 5.4 kbps loss.
The embodiments described herein address the above inefficiency problems by allowing the system to schedule multiple users within a transmission slot rather than just one user per slot, which is the case in a typical TDMA system. By utilizing CDMA techniques on a TDMA slot structure, multiple users will be scheduled to occupy the xe2x80x9cfat pipexe2x80x9d to optimize system throughput. The embodiments will describe methods and apparatus for selecting transmit formats for each of the multiple users that are scheduled in a slot of a combined TDMA/CDMA system.
Methods and apparatus are presented herein to address the above stated needs. In one aspect, a method is presented for transmitting data from a base station to at least one remote station, the method comprising: determining a priority for each of the at least one remote station; using the priority for each of the at least one remote station to determine at least one transmit format; formatting a data payload into a message frame in accordance with a selected transmit format, wherein the selected transmit format is selected from the at least one transmit frame; and transmitting the message frame to the remote station.
In another aspect, a method is presented for selecting transmit formats for a plurality of simultaneous transmissions from a base station, wherein each of the plurality of simultaneous transmissions is for a different remote station, comprising: determining a priority level for each remote station; using the priority levels for each remote station within a revenue function; and selecting a transmit format for each of the plurality of simultaneous transmissions based upon a value of the revenue function.
In another aspect, a method is presented for selecting a transmit format for a transmission to a remote station, comprising: selecting a plurality of possible transmit formats for the remote station; determining which of the plurality of possible transmit formats for the remote station uses the fewest Walsh codes; formatting the transmission to the remote station in accordance with the transmit format with the fewest Walsh codes; if more than one of the plurality of possible transmit formats uses the fewest Walsh codes, then determining which of plurality of possible transmit formats that uses the fewest Walsh codes further uses the least amount of transmission power; and formatting the transmission to the remote station in accordance with the transmit format with the fewest Walsh codes and the least amount of transmission power.
In another aspect, a method is presented for scheduling simultaneous transmissions to a plurality of remote stations to optimize system throughput, comprising: determining all supportable transmit format sets for a first remote station and the plurality of remote stations; determining all unused system resources in accordance with each of the supportable transmit format sets; evaluating a revenue function using each of the supportable transmit format sets; if only one transmit format set satisfies a criterion of the revenue function evaluation, then transmitting simultaneously to the first remote station and the plurality of remote stations in accordance with the transmit format set that satisfies a criterion of the revenue function evaluation; if multiple transmit format sets from the supportable transmit format sets satisfy a criterion of the revenue function evaluation, then selecting one of the multiple transmit format sets based upon the number of Walsh codes; and transmitting simultaneously to the first remote station and the plurality of remote stations in accordance with the selected transmit format set.
In other aspects, various apparatus are configured to execute the method aspects described above. For example, in one aspect, a memory element and a processor configured to execute a set of instructions stored within the memory element, the set of instructions for: selecting a plurality of possible transmit formats for the remote station; determining which of the plurality of possible transmit formats for the remote station uses the fewest Walsh codes; formatting the transmission to the remote station in accordance with the transmit format with the fewest Walsh codes; if more than one of the plurality of possible transmit formats uses the fewest Walsh codes, then determining which of plurality of possible transmit formats that uses the fewest Walsh codes further uses the least amount of transmission power; and formatting the transmission to the remote station in accordance with the transmit format with the fewest Walsh codes and the least amount of transmission power.
In another aspect, a memory element and a processor configured to execute another set of instructions stored within the memory element are presented, the set of instructions for: determining all supportable transmit format sets for a first remote station and the plurality of remote stations; determining all unused system resources in accordance with each of the supportable transmit format sets; evaluating a revenue function using each of the supportable transmit format sets; if only one transmit format set satisfies a criterion of the revenue function evaluation, then transmitting simultaneously to the first remote station and the plurality of remote stations in accordance with the transmit format set that satisfies a criterion of the revenue function evaluation; if multiple transmit format sets from the supportable transmit format sets satisfy a criterion of the revenue function evaluation, then selecting one of the multiple transmit format sets based upon the number of Walsh codes; and transmitting simultaneously to the first remote station and the plurality of remote stations in accordance with the selected transmit format set.